1. Field of the Invention
The present invention relates to a mask and, more particularly, to an X-ray mask and its fabrication method thereof.
2. Discussion of Related Art
Optical lithography exposure techniques have reached their technical and economical limitations as current leading techniques in the semiconductor industry. As a substitute for the exposure techniques, an X-ray lithography technique is suggested. In the X-ray lithography technique, the most important factor is the development of an X-ray mask.
FIGS. 1A to 1D are cross-sectional views illustrating a process for fabricating an X-ray mask according to a conventional subtractive method. In the conventional subtractive method, the X-ray mask is fabricated by forming an X-ray absorber pattern first and then forming a resist. The resist is treated using an e-beam, i.e., electron beam, lithographic technique to form a resist pattern.
Referring to FIG. 1A, a thin membrane 11 is formed on a silicon substrate 10. The thin membrane 11 is formed from a material such as silicon carbide (SiC), silicon nitride (SiN), or diamond. An etching mask film is then deposited on a bottom surface of the silicon substrate 10 and patterned to form an etching mask 12. The etching mask 12 is used to expose a designated portion of the silicon substrate 10. The exposed designated portion of the silicon substrate 10 is anisotropically etched using a silicon etching solution such as KOH. By using the etching mask film 12, a trench area 18 is formed. The trench area 18 allows for an X-ray to pass through the X-ray mask in that area without passing through the silicon substrate.
Next, an X-ray absorber 13 is deposited on the membrane 11 using a thin layer deposition technique such as a sputtering technique. The X-ray absorber 13 is formed from one material of tungsten (W), tantalum (Ta) and tungsten titanium (W-Ti). Such materials have a high absorbtivity characteristic for an X-ray.
As shown in FIG. 1B, a hard mask 14 is deposited on the X-ray absorber 13. A resist is formed on the hard mask 14 and patterned using e-beam lithography technique to form a resist pattern 15 that has a designated shape. After forming the resist pattern 15, the hard mask 14 is then exposed.
Referring to FIG. 1C, the hard mask 14 is selectively removed using a dry etching technique that uses the resist pattern 15 as a mask. Thus, a hard mask pattern 14' is formed that allows for the X-ray absorber 13 to be exposed.
As shown in FIG. 1D, the exposed portions of the X-ray absorber 13 are etched using the hard mask pattern 14' as a mask. Thus, portions of the X-ray absorber 13 underneath the hard mask pattern 14' remain along with the hard mask pattern 14'. As a result, an X-ray absorber patter 13' is formed thereby completing the X-ray mask.
FIGS. 2A-2D are cross-sectional views illustrating the process for fabricating an X-ray mask according to a conventional additive method.
In the conventional additive method, the X-ray mask is fabricated by selectively plating an X-ray absorber on a resist pattern which is previously formed.
Referring to FIG. 2A, a thin membrane 21 is formed on a silicon substrate 20. The thin membrane 21 is formed from a material such as silicon carbide (SiC), silicon nitride (SiN), or diamond. An etching mask film is then deposited on the bottom surface of the silicon substrate 20 and patterned to form an etching mask pattern 22 that exposes a designated portion of the silicon substrate 20. The exposed designated portion of the silicon substrate 20 is anisotropically etched using the etching mask pattern 22 to form a trench area 28. Like in the conventional subtractive method, the conventional additive method forms the trench area 28 that allows an X-ray to pass through the X-ray mask in that area without passing through the silicon substrate. Then, a seed metal layer 23 is formed on the membrane 21. The seed metal layer 23 is formed from Au/Cr.
As shown in FIG. 2B, a resist is then coated on the seed metal layer 23 and using e-beam lithography technique, a resist pattern 24 is formed having a designated shape. As shown in FIG. 2C, the resist pattern 24 is plated with a gold X-ray absorber 25, thus completing an X-ray mask.
The conventional methods for fabricating an X-ray mask have the following problems.
In the conventional subtractive method, when the resist is patterned with an electron beam, it is difficult to obtain an accurate pattern of the resist because of back scattering of the electron beam caused by the X-ray absorber that is previously formed. Also, another process is required for forming a hard mask between the absorber and resist to prevent the absorber from being removed by dry etching during the patterning of the resist since the etching rate of the resist is higher than that of the absorber. That is, without the hard mask, the absorber would also be etched as the resist is being etched.
In the conventional additive method, the electroplating process has difficulty controlling line widths below 0.18 microns. Also, the gold absorber has no minute structure due to its low density, thus deforming the mask by the incident X-ray into the absorber. The gold plated absorber of the conventional method has imperfections in the form of impurities, pores and grain boundaries. These imperfections lower the density of the gold plated absorber. The impurities and pores have thermal expansion characteristics that are different than intrinsic gold. When the gold plated absorber absorbs X-rays, the imperfections cause it to exhibit non-uniform thermal expansion. As a result, stress is generated in the gold plated absorber which deforms the pattern into which it is shaped.
The conventional method also requires the use of another process to remove the seed metal on the portion having no absorber thereon after the absorber is formed because the seed metal layer deteriorates the transparency to visible light, e.g., 633 nm wavelength of an He-Ne laser, of the X-ray mask.